The invention relates to magnetrons (e.g. planar magnetrons) such as are used in plasma sputtering systems.
The term magnetron refers to magnets that are placed behind the cathode (e.g. the target) in the plasma system. The magnets produce B fields in the plasma discharge in front of the target. The term planar means that the magnets are aligned in a plane parallel to the target surface.
The magnetron source acts to increase the electron density in the plasma discharge. The magnets that are placed behind the target generate magnetic field lines, a portion of which are somewhat parallel to the face of the target. These magnetic field lines, along with E fields of the electric circuit create forces acting on the electrons which tend to trap them next to the surface of the target. This increases the probability of collisions with the gas (e.g. argon) atoms thereby producing more gas ions to bombard the target.
If the magnets are placed behind the target, nonuniform erosion of the target is likely to occur. Indeed, the erosion pattern that appears on the face of the target will identify the locations of the magnets behind the target. To prevent such nonuniform erosion patterns from developing, the magnet assembly is typically moved over the backside of the target so as to produce on average a substantially uniform magnetic field over the entire face of the target from its center to its outer edges. Achieving uniform erosion is particularly desirable for a number of reasons including prolonging target life and producing uniform bottom coverage during deposition of metal into high aspect ratio contact holes. Additionally, uniform sputtering ensures that all regions of the target are continuously eroded, thereby preventing material from back sputtering onto non-active (i.e., non-sputtered) areas and then flaking off.
Since the magnetron produces an increased sputtering efficiency, the chamber pressure can be reduced as compared to a system which does not use a magnetron source. The electrons trapped adjacent to the front of the target along magnetic field lines spaced from the target make up for the smaller number of potentially ionized atoms present at low pressure by colliding with, and ionizing numerous of the gas atoms in the low pressure atmosphere. Low pressure operation can be particularly desirable because of its many positive benefits, including better contact hole-filling due to lower gas scattering of the sputtered material and improved film properties, e.g. lower resistivity, higher density, greater stability, less contamination, etc.
Low pressure operation and uniformity in target erosion, particularly at the target's edge are, however, competing objectives. Traditionally, low pressure operation is achieved by accepting less uniform target erosion. Similarly, improved uniformity in target erosion typically results in having to operate at higher chamber pressures.